Many types of cardiac arrhythmia, conditions in which the heart's normal rhythm is disrupted, are often treated by ablation (for example, radio frequency (RF) ablation, cryoablation, ultrasound ablation, laser ablation, microwave ablation, and the like), either epicardially or endocardially. Cardiac ablation may be performed with a variety of devices, such as devices having expandable distal ends. For example, ablation of the tissue surrounding a pulmonary vein ostium may be performed with a device having a distal end that is expandable into a substantially circular configuration, such as the pulmonary vein ablation catheter (PVAC®, Medtronic, Inc., Minneapolis, Minn.).
However, the success of any cardiac ablation procedure depends largely on the quality of the lesion(s) created during the procedure. Lesion quality depends, in turn, on the quality of contact between the ablation electrodes and the target tissue. Anatomical variations within the pulmonary veins, or other areas of target tissue, may cause a loss of contact between one or more electrodes and the target tissue.
Currently known methods for assessing contact between an electrode and tissue include visual contact assessment using fluoroscopic imaging. However, this method requires costly imaging equipment and can be time consuming. Other methods of electrode-tissue contact assessment involve monitoring the temperature of and power delivered to each electrode, but such methods are typically used after the delivery of ablation energy has already begun and often do not prevent unintended tissue damage. Additionally, these methods can be ineffective and subject to wide variation between patients.
As noted above, ablation may be applied via catheters designed to deliver amount of energy that optimize therapy yet minimize damage to surrounding tissue. A catheter used to deliver electrical energy to tissue has an inherent capacity of relaying electrical information to a remote impedance measurement device. If an impedance measurement is made pre- and post-ablation, an accurate assessment can be made as to the quality of the lesion. Therefore, the catheter's utility may be extended beyond its single role of delivering ablation into that of a lesion gauge, negating the need to introduce other gauges, cameras, or imaging systems to perform the same functions.
Difficulties arise when attempting to make impedance measurements via catheters, especially those with a high electrode count (for example, 16-electrode catheters). For impedance measurements resolved by a catheter, it is necessary to discern a bipolar impedance existing between electrodes from a unipolar impedance existing from a single electrode to a neutral or patient return electrode. Currently known systems and methods do not perform impedance disambiguation, with no discernment between unipolar and bipolar impedance and separation of these from parasitic impedances for multi-electrode catheters. Thus, a physician cannot attribute an impedance rending to the catheter, let alone to a specific location corresponding to catheter electrodes. As a result, the physician cannot infer lesion quality. Therefore, in order to imply lesion quality, the catheter and the impedance measurement system must “untangle” the multitude of impedance elements so as to provide a clear indicator of a particular catheter electrode's physiological effect.
An important function must then be to sift and accurately resolve a multitude of unipolar and bipolar impedance elements resulting from the many circuitous pathways resulting from a multi-electrode catheter placed inside the heart. Beyond the desired unipolar and bipolar results that can be related to their respective electrodes, there are additional undesired parasitic pathways that act to corrupt the desired impedance renderings. These pathways must also be measured, and dissected, or de-embedded from the desired unipolar and bipolar readings automatically. Undesired parasitic pathways include catheter wire series impedance, electric field (capacitive) coupling between catheter wires, and the placement of signal splitters and filters in-line with the catheter that divert catheter sensed electrogram (EGM) signals for use by an electrocardiogram (ECG) monitor.
Because of the potentially long duration required to collect electrical information necessary to disambiguate, calculate, and report unipolar and bipolar impedance over a large set of catheter electrodes, the system must have the ability to perform impedance measurements rapidly. To enable rapid impedance measurements, the system must apply that energy across all catheter electrodes simultaneously so that measurement results are correlated in time across frequency and electrode so that results are unaffected by slight movements of the catheter against tissue within the heart. Additionally, the physician must have a function that automatically scales the impedance measurement system's detector gain while maintaining the catheter electrodes' delivered current at safe, yet maximum, levels while the physician traverses widely ranging tissue and fluid impedances. Widely disparate tissue impedance could occur due to the difference of blood and heart tissue, ablated vs. non-ablated tissue, as well as from ice that forms an electrically insulting barrier on the tip of a cryoablation catheter, assuming that an impedance measurement function is used to support cryoablation lesion quality assessment.
Patient safety is a paramount concern, and delivery of measurement energy must be low enough in amplitude so as to meet applicable standards and possess a waveform that is biphasic so that inadvertent stimulation of cardio or nerve tissue does not occur. A further important consideration for an impedance measurement system is to provide an automatic, traceable calibration conforming to applicable standards so that the instrument's impedance renderings are trustworthy.
Finally, since the physician may use the impedance measurement function to determine lesion quality (gauged by ablation impedance renderings) immediately prior to and after the moment of therapy, the impedance measurement system must remain connected in-line with the catheter and the ablation generator simultaneously. Therefore, some mechanism must sequence and protect the impedance measurement function during the delivery of the high energy therapy.
It is therefore desired to provide a method and system for accurate and reliable electrode-tissue contact assessment, and for electrode-tissue contact assessment before the delivery of ablation energy begins.